1. Field of the Invention
The present invention relates to processing of semiconductor wafers such as slices of semiconductor silicon in electronic component fabrication and, more particularly, to an improved method and apparatus for planarizing the wafers in the chemical-mechanical planarization process in order to achieve a high degree of wafer planarity.
2. Problem to be Solved
In the manufacture of electronic components such as integrated circuits, wafer surface planarity is of extreme importance. Photolithographic processes are typically pushed close to the limit of resolution and it is essential that the wafer surface be highly planar so that the electromagnetic or other radiation used to create the integrated circuit may be accurately focused in a single level thus resulting in precise imaging over the entire surface of the wafer. Wavy, curved or wedge-shaped semiconductor disks result in lack of definition when, for example, a photosensitive resist is applied to the surface of the disk and exposed.
In order to achieve the degree of planarity required to produce ultra high density integrated circuits and other electronic component circuits, chemical-mechanical planarization processes are now typically employed in the industry. In general, the chemical-mechanical planariztion (CMP) process involves pressing a semiconductor wafer against a moving polishing surface that is wetted with a chemically reactive, abrasive slurry. Slurries are usually either basic or acidic and generally contain alumina or silica particles. The planarizing surface is typically a planar pad made of a relatively soft, porous material such as blown polyurethane. The pad is usually mounted on a planar rotatable platen but linear moving pads are also now being proposed as described below.
In general, the wafer is secured to a carrier plate (or wafer carrier)by a mounting medium such as an adhesive, with the wafer having a force load applied thereto through the carrier by a pressure plate so as to press the wafer into frictional contact with a planarizing pad mounted on a rotating or linear moving turntable. The carrier and pressure plate also rotate as the result of either the driving friction from the rotating turntable or rotation drive means directly attached to the pressure plate for a rotary turntable or linear turntable or linear moving pad.
In a typical planarization machine, the movement of the carrier is programmed to acquire a wafer from a first station, to transport the wafer to a planarizing surface, to drive the wafer across the rotating planarizing surface, to transport the wafer from the planarizing surface to a second station, and to release the wafer at the second station. A typical way of securing and releasing the wafer is by the use of a vacuum head that includes a rigid perforated plate against which the wafer is drawn by applying a vacuum to a plenum lying above the perforated plate.
During planarization it has been found that when a force is imposed on the wafer particularly when using a rotating pad and turntable the planarizing action across the wafer is not uniform causing center-to-edge non uniformity in thickness and poor flatness of the wafer. The surface life of the planarizing pad is also a factor in affecting the planarity of the planarized wafer. Frictional heat generated at the wafer surface enhances the chemical action of the planarizing fluid and thus increases the planarization rate. The frictional heat however can cause planarity problems unless the heat is evenly transmitted over the surface of the wafer and typical planarizing systems utilize cooling systems to control the temperature of the planarization operation.
A number of attempts have been made in the prior art to improve the planarity of CMP operations. In U.S. Pat. No. 4,270,316 the uneven transmission of pressure which causes different degrees of abrasion of the planarized disks is compensated for by the provision of soft elastic inserts placed between a pressure piston and the back of the carrier plate on which the disks to be planarized are cemented. In U.S. Pat. No. 4,313,284 a deformable thin disk carrier is mounted through a resilient device to a rotatable pressure plate so that the carrier can be deformed to either a concave shape or convex shape depending on the planarization required. In U.S. Pat. No. 4,910,155 a dam is provided on the planarizing plate so that the planarizing pool of slurry completely immerses the planarizing pad. In U.S. Pat. No. 4,918,869 the use of pressurized air acting on the pressure plate is provided so that the pressure on the wafer surface can be uniform. In U.S. Pat. No. 5,036,630 the wafer carrier comprises at least two (2) materials having different coefficients of thermal expansion which carrier imparts a desired convex or concave bias to the wafer during the planarizing operation. In U.S. Pat. No. 5,423,716 the lower face of the backing plate of the wafer carrier includes a number of recessed areas to which a vacuum can selectively be applied. The vacuum is applied to suck a resilient membrane into the recessed areas to draw the wafer into position. The same apparatus can be used to apply a pressurized fluid to the wafer to exert a uniform downward pressure on the wafer. In U.S. Pat. No. 5,486,129 the pressure head of the wafer carrier contains a number of pressure applicators over the wafer surface which can be monitored and adjusted to vary the pressure on the wafer during the planarizing operation.
In U.S. Pat. No. 5,486,265 uniform chemical-mechanical planarization is achieved at a high material removal rate by pulsing the pressure applied to the wafer undergoing planarization. The pressure is pulsed between an initial optimum pressure and a reduced second pressure, preferably about 0 psi so that the cleaning slurry reaches all portions of the wafer surface and eliminates the negative impact of starvation areas which do not have a sufficient amount of cleaning slurry.
In U.S. Pat. No. 5,522,965, a chemical-mechanical planarization method is disclosed wherein a non-rotating planarization pad is used and energy, e.g., ultrasonic energy, applied to the pad to aid in the removal of surface material from the wafer and for pad conditioning.
The disclosures of the above patents are hereby incorporated by reference.
The above described CMP process was mainly directed to the methods typically used in the industry today which are basically termed a rotary or orbital polishing technique. The limitations of such rotary or orbital techniques are becoming increasingly evident since the wafer is inherently exposed to unequal radial velocities on its surface during polishing. These velocities which increase along the radius of the polishing platen and pad cause removal rates to vary across the wafer surface.
The next generation CMP process may be a non-rotary type technique now labeled Linear Planarization Technology (LPT). This technique uses a linear belt polish pad and eliminates the unequal radial velocities encountered during orbital or rotary polishing. Such techniques as shown in an article entitled Linear Planarization Technology published by OnTrak Systems, Inc. LAM Research also describes this technique in an article entitled Teres.TM. CMP System Linear Planarization Technology(LPT) dated February 1998. In the LPT technology and in the rotary (orbital) technology, polishing pressure may be applied from underneath the polishing pad via fluid flow (air or water) or from above the polishing pad as is typically used in the rotary and orbital processes.
For convenience, the following description will be directed to rotary CMP processes and to such processes where a polishing pressure is exerted on the wafer from above the pad but it will be appreciated to those skilled in the art that the method and apparatus of the invention may be used for other CMP processes.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved apparatus, e.g., CMP apparatus, for planarizing semiconductor wafers and other workpieces.
It is another object of the present invention to provide an improved method for planarizing workpieces, e.g., wafers, using such planarizing devices as a CMP apparatus.
It is a further object of the invention to provide flat workpieces, including planarized semiconductor wafers, made using the improved method and apparatus of the invention.
Other objects and advantages of the present invention will be readily apparent from the following description.